During the course of standardization up to Release 11, a number of features were standardized to autonomously improve/optimize mobility, including load balancing, coverage, and capacity. Some of these features include Mobility Robustness Optimization (MRO) and Mobility Load Balancing (MLB).
In the course of Release 12, a new Study Item was started. As described in RP-122027, active antennas may allow the creation of multiple vertical and horizontal beams making deployment dynamic thereby enabling dynamic cell splitting/merging to handle changing load conditions. For example, beams may be steered to distribute capacity precisely according to actual traffic mix, traffic location, and/or user demands. Active antennas may thus be particularly useful for suburban and rural areas, where fixed deployment of pico cells is expensive, but the network may face congestion situations nonetheless. SON can automate the network deployment using active antennas. These scenarios may open up discussions on how to design new solutions that would enable a Self Optimized Network (SON) to automatically adjust to the changes introduced by Active Antenna Systems (AAS).
LTE
FIG. 1 illustrates a Long Term Evolution (LTE) architecture according to the Third Generation Partnership Project (3GPP) including logical interfaces (X2 interfaces) between base stations (also referred to as evolved nodeBs or eNBs) 101a, 101b, and 101c and logical interfaces (S1 interfaces) between each eNB and a respective packet core node (also referred to as a Mobility Management Entity Serving Gateway or MME/S-GW) 103a/103b. As shown, S1 interfaces may connect eNBs to MME/S-GWs, while X2 interfaces may connect peer eNBs.
An Evolved Universal Terrestrial Radio Access Network (E-UTRAN) includes a plurality of eNB nodes, which are connected to each other via an X2 interface(s). The S1 and the X2 interfaces can be divided into control plane (dashed lines) and user plane (solid lines) parts. While E-UTRAN is discussed by way of example, embodiments disclosed herein may be applied in other networks/standards (e.g., GSM, UTRAN, etc.). E-UTRAN is discussed herein merely by way of example.
Network Management
An example of a management system is shown in FIG. 2. The node elements or NEs (also referred to as base stations, eNodeBs, eNBs, etc.) are managed by a domain manager or DM (also referred to as the Operation and Support System or OSS). A DM is sometimes seen as comprising an element manager (EM), and/or sometimes, the EM is considered to be embedded in an NE. A DM may further be managed by a network manager (NM). An interface between two NEs may be provided using an X2 interface, whereas an interface between two DMs is referred to as an Itf-P2P interface. The management system may configure the network elements NEs, as well as receive observations associated with features in the network elements. For example, a domain manager DM may observe and configure network elements NEs, while a network manager NM observes and configures domain managers DMs. A network manager NM may also observe and configure network elements NEs via domain manager(s). According to embodiments disclosed herein, any function that automatically optimizes/improves NE parameters can in principle execute in the NE(s), DM(s), and/or the NM(s). Using configuration via a domain manager(s) DM(s), network manager(s) NM(s), and/or related interfaces, functions over the X2 and/or S1 interfaces may be coordinated throughout the radio access network (RAN), eventually involving the core network (i.e., the MME/S-GW).
Handover in LTE
FIGS. 3A and 3B illustrate X2 Handover in LTE in accordance with 3GPP TS 36.300, version 11.4.0, entitled “E-UTRAN overall description.”
Handover is a significant aspect of any mobile communication system where the system tries to provide service continuity of wireless terminals (also referred to as User Equipment nodes, user equipment, UE, etc.) by transferring the connection of a UE from one cell to another depending on factors such as relative signal strengths, load conditions, service requirements, etc. The provision of efficient/effective handovers (reduced/minimum number of unnecessary handovers, reduced/minimum number of handover failures, reduced/minimum handover delay, etc.), may affect not only the Quality of Service (QoS) of the end user but also overall mobile network capacity and/or performance.
In LTE (Long-Term Evolution), UE-assisted, network controlled handover may be used. In such systems, the network configures the UE to send measurement reports, and based on these reports, the UE is moved (if useful/required and if possible) to an appropriate cell that will provide service continuity and/or quality. Handover may be performed via an X2 connection, whenever available, and if not, using an S1 connection (i.e., involving the Core Network or CN). The X2 Handover process is shown in FIGS. 3A and 3B. The handover procedure can be sub-divided into three stages of preparation (initiation), execution, and completion.
Based on measurement results the source eNB receives from the UE during the preparation stage (e.g., using measurement control signaling 3-1 and measurement reports 3-2), the source eNB decides whether to handover the connection to another eNB (referred to as a target eNB) or not (e.g., HandOver Decision 3-3). If the decision is made to handover, the source eNB sends a HANDOVER REQUEST message 3-4 to the target eNB.
If the target eNB is able to admit the UE (e.g., Admission Control 3-5 and HandOver Request ACK 3-6), a message is sent to the UE (RRC Connection Reconfiguration Mobility Control Information 3-7) to initiate the handover, and the handover execution stage is entered. Downlink (DL) data arriving at the source eNB for the UE is then forwarded to the new target eNB.
The handover completion stage is entered once the target eNB and the UE are synchronized (SN Status Transfer 3-1, Synchronization 3-9, and/or UL Allocation and TA 3-10) and a handover confirm message (RRC Connection Reconfiguration Complete 3-11) is received by the target eNB. After a proper setup of the connection with the target eNB is performed (including Path Switch Request 3-12, Modify Bearer Request 3-13, and switching of the DL path 3-14 in the serving gateway), the old connection is released (Modify Bearer Response 3-15, Path Switch Request 3-16, UE Context Release 3-17, and Release Resources 3-18) and any remaining data in the source eNB that is destined for the UE is forwarded to the target eNB. Then normal packet flow can ensue through the target eNB.
Handover Measurement Triggering
A UE measurement report configuration includes the reporting criteria (whether it is periodic or event triggered) as well as the measurement information that the UE has to report. The following event-triggered criteria are specified for intra-RAT (intra Radio Access Technology) measurement reporting in LTE:
Event A1, Serving cell becomes better than absolute threshold;
Event A2, Serving cell becomes worse than absolute threshold;
Event A3, Neighbor cell becomes better than an offset relative to the serving cell;
Event A4, Neighbor cell becomes better than absolute threshold; and
Event A5, Serving cell becomes worse than one absolute threshold and neighbor cell becomes better than another absolute threshold.
Issues Related to Handover
As the brief description relating to Handover Measurement Triggering indicates, handover in LTE (Long Term Evolution) is controlled via several parameters. Incorrect parameter settings can lead to problems such as Radio Link Failure (RLF), Handover Failure (HOF), and/or Ping-pong Handover (also known as Handover Oscillation).
Radio Link Failure
If the handover parameters are set in such a way that the UE does not report handover measurements on time, the UE may lose the connection with the original cell before handover is initiated. As described in 3GPP TS 36.331 version 11.2.0, entitled “Radio Resource Control,” when the UE receives a certain number of (N310) consecutive “out of sync” indications from the lower layer, the UE assumes a physical layer problem is ensuing, and a timer (T310) is started. If the UE doesn't receive a certain number of (N311) consecutive “in sync” indications from the lower layer before timer T310 expires, RLF is detected. RLF is also detected when a random access problem is indicated from MAC or upon indication that the maximum number of RLC retransmissions has been reached.
Handover Failure
Handover Failure (HOF) occurs if the connection with the original cell is lost while HO is ongoing with the target. When the UE receives a HO command (i.e. RRCConnectionReconfigurationRequest with mobilityControlInfo, as shown in FIGS. 3A and 3B), it starts a timer (T304), and if this timer expires before the HO is completed (i.e. RRCConnectionReconfigurationComplete message is sent by the UE), a HOF is detected.
Ping-Pong Handover/Handover Oscillation
Improper setting of handover parameters can make the UE handover back and forth between two neighboring cells. An example of this is a setting that makes the triggering conditions for the handover events (e.g., A3) valid between the source and neighbor cells at the same time. FIG. 4 illustrates ping-pong handover (also referred to as handover oscillation). A UE is said to have experienced handover oscillation if it stays in a target cell (CellB) for duration (T) that is less than the handover oscillation threshold (TOSC), before it is handed back to the source cell (CellA). The oscillation rate can be defined as a ratio between the number of oscillations and the total number of HOs (HandOvers).
There is an upper boundary for an acceptable oscillation rate originating, for example, from core network load. Also the oscillation rate is related to end-user performance. On one hand, oscillation may be harmful because it may induce additional signaling and/or delays, and on the other hand, oscillations may allow the user to be connected to the best cell. This should be balanced for the end-user to experience a desired level of performance.
RRC Connection Reestablishment
When a RLF or HOF is detected by the UE, the UE starts a timer (T311) and tries to re-establish the connection to the best available cell (e.g., the source cell, another cell belonging to the same source eNB or a neighbor cell belonging to another eNB). This is known as RRC (Radio Resource Control) Connection Reestablishment, and is shown in FIG. 5. The UE transmits an RRCConnectionReestablishmentRequest 5-1 to the EUTRAN node, the EUTRAN node transmits an RRCConnectionReestablishment message 5-2 to the UE, and the UE transmits an RRCConnectionReestablishmentComplete message 5-3. The UE includes the following information in the re-establishment request: Physical Cell ID (PCI) of the last cell the UE was connected to before RLF; UE Identity including the C-RNTI (Cell Radio-Network Temporary Identifier) as well as MAC ID (Medium Access Control Identifier) for context lookup, using which the last serving cell can identify the UE; and Re-establishment cause (e.g., whether the request is due to handover failure, reconfiguration failure, or other causes).
If the UE context is found in the cell (if it is the source cell or if it was a cell prepared for handover, i.e., handover was ongoing when the RLF happened and the cell where the UE re-appeared already has the UE context, which was communicated to it from the source cell during Handover Request message exchange), the connection is re-established. Otherwise (if UE context is not available, or re-establishment did not succeed before T311 expires), the UE goes to IDLE mode and tears down all the active bearers, if any, and may restart the bearer setups if needed.
Mobility Robustness Optimization (MRO)
Configuring all the HO parameters manually to reduce/avoid occurrence of the aforementioned problems may be too expensive and can be challenging. As such, Mobility Robustness Optimization (MRO) has been introduced in 3GPP to automate the dynamic configuration of handover parameters. Briefly, MRO tries to identify the following three situations (too late HO, too early HO, HO to wrong cell), and based on the statistical occurrence of these situations, MRO tries to adjust the HO parameters.
Too Late HO occurs when a UE is handed over late to the target cell, so that the link to the source cell breaks before completing the handover.
Too Early HO occurs when a UE is handed over to a candidate cell too early resulting in a radio link or handover failure in the target cell. The UE returns soon to the source cell via re-establishment procedures.
Handover to wrong cell occurs when a UE is handed over to one target cell but it experiences a RLF within a short duration after that in the target cell and the UE re-establishes the connection at another cell. A proper parameter setting would most probably have led to the handing over of the UE to the last target cell to begin with.
MRO tries to gather statistics on the occurrence of Too Late HOs, Too Early HOs, and HO to the wrong cell, and these statistics can be used to adjust the handover parameters. One or more of the following handover parameters controlling the event driven reporting of the UE can be adjusted using MRO: Threshold indicating how much stronger a certain candidate cell needs to be before it is reported to the serving cell; Filter coefficient(s) applied to the measurement before evaluation triggers are considered; and/or Time to trigger meaning the time window within which the triggering condition needs to be continuously met in order to trigger the reporting event in the UE. For example, a higher ‘too early handover’ ratio than desired can be counter-acted by increasing the threshold, thereby delaying the triggering of A3 event. Another example could be the resolving of a higher ‘handover to wrong cell’ ratio than desired by increasing the threshold towards the first, unwanted, target cell.
Three main message types (i.e., RLF reports between the UE and eNBs, RLF INDICATION reports between eNBs, and HANDOVER REPORTs between eNBs) are used by MRO to communicate/gather information regarding Too Early Handover, Too Late Handover, and Handover to the wrong cell. HandOver HO failures are discussed, for example, in Section 22.4.2 of TS 36.300, V.11.4.0 (2012-12), “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 11), pages 1-208. Moreover, the disclosure of TS 36.300, V.11.4.0 is hereby incorporated herein in its entirety by reference.
Mobility Load Balancing
As specified in 3GPP TS 36.300 version 11.4.0, entitled “E-UTRAN overall description,” the objective of load balancing is to distribute cell load evenly among cells and/or to transfer part of the traffic from congested cells. This may be performed using self-optimization of mobility parameters and/or handover actions.
Self-optimization/improvement of the intra-LTE and inter-RAT mobility parameters to the current load in the cell and in the adjacent cells can improve system capacity compared to static/non-optimized cell reselection/handover parameters. Such optimization/improvement can also reduce/minimize human intervention in the network management and optimization/improvement tasks. As shown in FIG. 6, support for mobility load balancing within a radio access network RAN using load balancing algorithm 601 may include one or more of following functions:
Load reporting 603;
Load balancing 605 action based on handovers; and/or
Adapting handover 607 and/or reselection configuration.
Triggering of each of these functions is optional and may depend on implementation. A functional architecture of SON (Self Optimized Network) load balancing is presented in FIG. 6.
The Mobility Settings Change is a procedure used to induce load balancing via mobility and/or to report and suggest changes of the parameters used to trigger mobility between two nodes. A mobility setting change procedure is illustrated in FIG. 7 where an X2-AP Mobility Change Request is transmitted 7-1 from eNB-1 to eNB-2, an X2-AP Mobility Change Acknowledge is transmitted 7-2 from eNB-2 to eNB-1, and an X2-AP Mobility Change Failure is transmitted 7-3 from eNB-2 to eNB-1.
Active or Adaptive or Reconfigurable Antenna Systems
Antenna systems in mobile communications may be based on a combination of one or more antenna elements. The antenna elements of the system can be combined in different ways (e.g., using different amplitudes, delays/phases, frequencies, etc.) to focus the antenna transmission and reception with directivity. The antenna system can transmit and receive more energy in some directions than in others. By reconfiguring/adapting the combination of antenna elements, the antenna system can be adapted to change directivity over time. This means that it is possible to change the cell shape from one cell shape to another at a specific time, where cell shape reflects the area in which the cell associated with the antenna system is providing services. An antenna system, for example, can include an array of antenna elements supporting transmission to and reception from a cell (also referred to as a sector), and a base station may include three such antenna systems supporting communications over three 120 degree cells/sectors. A cell is further associated with a radio access technology and a frequency carrier for downlink communication and a frequency carrier for uplink communication. The uplink and downlink frequency carriers can be different as in frequency division duplex (FDD) or the same as in time division duplex (TDD). The antenna system can provide service to multiple cells covering a similar sector area. In other nomenclatures, this could also be considered as different antenna systems, with a one to one mapping between the cell and the antenna system. Even though one antenna system is associated with multiple cells covering a similar sector area, it can still be configured differently for different cells, also possibly differently for uplink and downlink operations.
In some current base stations, a single radio transceiver drives a group of antenna elements. The signal from the transceiver to each element is delayed by a differing amount, such that the phase of the signal transmitted at each antenna elements differs. The phase difference impacts the direction of radiation of the antenna. Typically, the transceiver may be located some distance from the antenna elements, with connection therebetween provided via a cable. In so-called MIMO systems, there may be several transceivers, but the transceivers may not be integrated with the antennas.
Passive antenna systems may be reconfigurable, for example, using a Remote Electrical Tilt, so that phase delay paths between transceiver and elements are physically adjusted.
In an active antenna system, the transceivers and antennas may be in general be integrated to some extent. Furthermore, instead of one transceiver driving all elements, several transmitters may drive one or more elements in groups. Unlike MIMO systems, the transceiver to element mapping may allow dynamic adjustment of beams transmitting the same reference symbols.
The term Active Antenna System may refer to base stations that integrate radio and antenna elements, or that have multiple transceivers mapped to different elements but transmitting the same Cell specific Reference Signal (CRS), or both.
By reconfiguring/adapting amplitude and phase of AAS transceivers, the antenna system can be adapted to change directivity over time. This means that it is possible to change the cell shape from one cell shape to another at a specific time. Accordingly, it may be possible to change the cell shape from one cell shape to another cell shape at a specific time, where cell shape reflects the area in which the cell associated with the antenna system is providing services.
In general, an antenna system can be used to realize/provide/service one or more cells, and AAS beamforming operation also can include splitting and merging of cells. AAS operation can be managed by the associated base station, or by a different network or management node. For example, one antenna system for a 120 degree cell/sector may be configured to adaptively split the cell/sector (having one cell/sector identification) into two or more smaller cells/sectors (each having respective different identifications), to merge two smaller cells/sectors into one larger cell/sector, to change a shape/size/range of the cell(s)/sector(s), etc. AAS is sometimes denoted adaptive antenna systems or reconfigurable antenna systems (RAS) or antenna arrays or group antennas. As used herein, the term adaptive antenna system denotes any antenna system (including an Active Antenna System or AAS) that can be reconfigured/adapted while in operation as well as when not in operation.
Procedures to optimize/improve mobility robustness, load balancing, coverage, and/or capacity have generally been designed on the basis of semi-static deployment scenarios. Namely, scenarios considered may not change dynamically, but rather may be stable until the optimization function decides to apply a corrective measure, for example, to change mobility parameters and/or cell coverage.
With the adoption of dynamic AAS based solutions it may be possible that the cell deployment topology changes in a dynamic manner. Such dynamic changes at one cell introduced by AAS (e.g., cell splitting, cell merging, cell expansion/contraction, etc.), however, may be incompatible with current SON applications.